1. Field of the Invention
The present invention relates to a wavelength-tunable stabilized laser which can vary the wavelength of emitted laser light and which, in particular, can almost lock the wavelength of emitted laser light at a desired wavelength.
To construct multimedia networks in the future, ultralong distance, large-capacity optical communication apparatuses are now demanded. To realize such large capacity, wavelength division multiplexing (WDM) optical communication apparatuses are studied and developed because of their advantages such as capability of effectively utilizing the wide bandwidth and large capacity of the optical fiber.
In particular, WDM light source for WDM optical communication is required to output laser light at a plurality of wavelengths. Further, the wavelength spacing need to satisfy a standard, for example, the grid spacing defined for respective channels according to the ITU-T recommendation. WDM light source is studied and developed so as to satisfy such a requirement.
2. Description of the Related Art
Conventionally, in the case of WDM optical communication systems that perform communication by using a four-wave WDM optical signal, a WDM light source is provided with four semiconductor lasers that emit laser beams having different wavelengths or a single wavelength-tunable laser capable of varying the oscillation wavelength by changing the device temperature or the driving current. For example, the multiple quantumwell (MQW) DFB laser, the wavelength-tunable distributed Bragg reflection (DBR) laser, or the like is used as the wavelength-tunable laser.
In particular, the use of a wavelength-tunable laser provides an advantage that the number of semiconductor lasers used as regular-use light sources and reserve light sources are reduced in a WDM light source. For example, in a 32-wave WDM optical communication system, 32 semiconductor lasers are necessary as each of regular-use light sources and reserve light sources if one semiconductor laser is used for one wavelength. In contrast, where wavelength-tunable lasers each capable of emitting laser beams of four wavelengths are used, it is sufficient to use eight semiconductor lasers as each of regular-use light sources and reserve light sources (maximum case).
On the other hand, although in semiconductor lasers the diffraction grating pitch etc. are so designed that single-mode laser light having a predetermined wavelength is emitted in a steady state, oscillation at the predetermined wavelength does not necessary occur at the time of ignition. Even in a steady state, the oscillation wavelength is not always locked at the predetermined wavelength owing to fluctuations. In the case of wavelength-tunable lasers, which are also associated with the above phenomena, the oscillation wavelength needs to be locked at a targeted predetermined wavelength because they can oscillate at multiple wavelengths.
To lock the oscillation wavelength at a desired wavelength, a wavelength locking apparatus is used in WDM light sources.
Referring to FIG. 21A, laser light output from a wavelength-tunable laser 911 is input to a coupler (CPL) 912 that is provided in a multi-wavelength locking apparatus 905 and serves to branch input light into two parts. One branched laser light is output as output light of a WDM light source. The wavelength-tunable laser 911 is an MQW semiconductor laser and has, for example, a characteristic that the oscillation wavelength varies by 0.8 nm when the device temperature is changed by about 8xc2x0 C.-10xc2x0 C. Where a WDM optical signal contains four optical signals at wavelength spacing of 0.8 nm according to the ITU-T recommendation, the wavelength-tunable laser 911 can output laser beams of four wavelengths in a temperature range of about 30xc2x0 C. and emits laser light having one of the four wavelengths by controlling the device temperature.
In the multi-wavelength locking apparatus 905, the other laser light that has branched off at the coupler 912 is input to a coupler 913 for branching input light into two parts. One laser light that has branched off at the coupler 913 is input, via a Fabry-Pxc3xa9rot etalon filter (ET filter) 914, to a first photodiode (PD) 915 for outputting a current in accordance with light intensity. The light intensity of the laser light is detected by the first photodiode 915. The output value of the first PD 915 is represented by PDo1. The other laser light that has branched off at the coupler 913 is input to a second PD 916, where its light intensity is detected. The output value of the second PD 916 is represented by PDo2.
In the ET filter 914, wavelengths having extremum transmittance values are so set that the PDo1 value as normalized by the PDo2 value at an intended locking wavelength, that is, PDo1/PDo2, becomes a target value 0.5.
A control CPU 917 receives PDo1 and PDo2. The control CPU 917 generates a control signal to be used for locking the oscillation wavelength of the wavelength-tunable laser 911 based on these detection values and sends it to the wavelength-tunable laser 911.
The WDM light source having the above configuration operates in the following manner and thereby locks the oscillation wavelength of the wavelength-tunable laser 911 at a ch0 wavelength, for example.
After igniting the wavelength-tunable laser 911, the control CPU 917 receives PDo1 and PDo2 and calculates PDo1/PDo2 (see FIGS. 21A and 21B). When PDo1/PDo2 is greater than the target value 0.5, the control CPU 911 controls the wavelength-tunable laser 911 so that the oscillation wavelength becomes longer by adjusting its device temperature. On the other hand, when PDo1/PDo2 at the time of the ignition is smaller than the target value 0.5, the control CPU 917 controls the wavelength-tunable laser 911 so that the oscillation wavelength becomes shorter. The wavelength-tunable laser 911 is controlled in this manner so that PDo1/PDo2 is always kept at 0.5 and its oscillation wavelength is thereby locked at the ch0 wavelength.
Where the control CPU 917 controls the oscillation wavelength merely by performing the magnitude comparison between PDo1/PDo2 and the target value 0.5, the oscillation wavelength can be locked at the desired ch0 wavelength when the wavelength-tunable laser 911 has ignited at a wavelength of any of points a-d in FIG. 21B. However, when the wavelength-tunable laser 911 has ignited at a wavelength of point e or f, the oscillation wavelength is locked at a wavelength other than the ch0 wavelength.
In view of the above, the control CPU 917 also controls the device temperature at the time of ignition in consideration of a range including the oscillation wavelength at the time of ignition of the wavelength-tunable laser 911.
As described above, in the WDM light source, the oscillation wavelength can be locked at any of ch1, ch2, and ch3 wavelengths in the same manner as at the ch0 wavelength by taking the device temperature at the time of ignition into consideration.
For a wavelength at the time of laser ignition, a wavelength range where a wavelength locking apparatus can lock the laser oscillation wavelength at a desired wavelength is referred to as xe2x80x9clocking range.xe2x80x9d
The locking range width is determined by the FSR (free spectral range) of the ET filter 914 because PDo1/PDo2 has the same value as the wavelength shifts by the FSR as shown in FIG. 21C. Therefore, to equalize the oscillation wavelengths to the wavelengths of optical signals of a WDM optical signal, the FSR of the ET filter 914 is set equal to the wavelength spacing of the WDM optical signal.
On the other hand, the transmittance-wavelength characteristic of the ET filter 914 depends on the temperature. As seen from FIG. 22, as the temperature increases, the transmittance-wavelength characteristic shifts to the longer wavelength side in parallel with the horizontal axis at a rate of about 0.095 nm/xc2x0C. In FIG. 22, the vertical axis represents the current value in xcexcA (corresponding to the transmittance) of a detector and the horizontal axis represents the wavelength in nm.
The ET filter was made of quartz glass, the mirror surface reflectance was 25%, and the measurement temperature was 22.1xc2x0 C., 30.0xc2x0 C., 37.9xc2x0 C., and 45.7xc2x0 C.
Because of rapid increase of the communication capacity, it is now required to increase the multiplexing number of a WDM optical signal. To this end, WDM light sources are required to be able to oscillate at many wavelengths such as 32 or 64 wavelengths. On the other hand, since the variable range of the device temperature of wavelength-tunable lasers is limited, WDM light sources need to incorporate a plurality of wavelength-tunable lasers.
In the above circumstances, multi-wavelength locking apparatuses need to attain locking at a desired wavelength for all wavelengths at which oscillation is possible. However, since the transmittance-wavelength characteristic of the EF filter in the apparatus depends on the temperature, the conventional configuration cannot attain locking at all wavelengths.
In particular, locking all wavelengths is more difficult when a wavelength-tunable laser in which the oscillation wavelength is changed by a temperature control, is integrated with an ET filter.
Assume a case where an 8-wave WDM light source is composed of a wavelength-tunable laser capable of oscillating at 0ch to 3ch wavelengths and a wavelength-tunable laser capable of oscillating at 4ch to 7ch wavelengths and an ET filter is made of quartz glass. The each pair of 0ch and 4ch, 1ch and 5ch, 2ch and 6ch, and 3ch and 7ch are controlled at the same device temperature. The transmittance-wavelength characteristic of the ET filter has temperature dependence that the curve shifts by about 0.01 nm for a temperature variation of 1xc2x0 C. When the wavelength spacing of a WDM optical signal is set at 0.8 nm, the device temperature needs to be changed by 10xc2x0 C. for each channel, in which case the transmittance-wavelength characteristic of the ET filter shifts by 0.1 nm.
In this case, in the multi-wavelength locking apparatus, the device temperature is set at the same temperature for 0ch and 4ch, for example, so that the FSR of the ET filter needs to be set at 0.8 nm as described above to lock laser beams at the 0ch and 4ch wavelengths. On the other hand, to lock laser beams at the 0ch to 3ch wavelengths, the FSR of the ET filter needs to be set at 0.7 (=0.8xe2x88x920.1) nm in view of the shift of the ET filter.
That is, in the multi-wavelength locking apparatus, the FSR of the ET filter needs to be set at 0.8 nm on one hand and needs to be set at 0.7 nm on the other hand. It is difficult to satisfy both requirements as long as the conventional configuration is employed.
In addition, when it is attempted to increase the transmission rate from 2.5 Gbps to 10 Gbps to increase the communication capacity in the conventional configuration, increased modulation rate causes a spectrum line width problem and a relative intensity noise problem.
An object of the invention is to provide a wavelength-tunable stabilized laser capable of changing and stabilizing the wavelength of laser light.
The above object is attained by a wavelength-tunable stabilized laser comprising: a light source comprising a plurality of lasers capable of oscillating at a plurality of wavelengths; a light detecting part for detecting the light intensity of laser light output from the light source via a periodic filter; and a controlling part for generating oscillation of one of the lasers of the light source and controlling the oscillation wavelength of the laser so that an output value of the light detecting part becomes equal to a predetermined one of a plurality of target values.
In the invention, a plurality of target values is respectively set for each of wavelengths at which oscillation is possible. This makes it possible to generate oscillation of laser light at a desired wavelength among the plurality of wavelengths and stabilize the laser light at the desired wavelength. Further, integrating wavelength-tunable lasers and a wavelength detecting part realizes reduction in size and cost of the wavelength-tunable stabilized laser according to the invention.